Cold-proof grease composition for reducing noise

ABSTRACT

The invention provides a cold-proof grease composition for reducing noise. In certain embodiments, the invention provides a grease composition having an improved noise-reduction effect without sacrificing lubricant adhesiveness at high and low temperatures. In one embodiment, the grease composition comprises a synthetic hydrocarbon oil as a base oil, a lithium-based thickener, and a poly(methyl acrylate) (PMA)-based copolymer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to Korean PatentApplication No. 10-2012-0120174, filed on Oct. 29, 2012, the disclosureof which is incorporated herein by reference in its entirety.

BACKGROUND

(a) Technical Field

The invention provides a cold-proof grease composition. In certainembodiments, the grease composition of the invention is useful forreducing noise. In particular, the grease composition of the inventioncomprises a synthetic hydrocarbon oil as a base oil, a lithium-basedthickener, and a poly(methyl acrylate) (PMA)-based copolymer. The greasecomposition of the invention offers an improved noise-reduction effectwithout sacrificing lubricant adhesiveness at high and low temperatures.

(b) Background Art

Grease is generally used as a lubricant for automotive components.Although a grease material having high viscosity helps to reduce noise,operation of the components is impaired due to an increased load.Consequently, an operation failure may occur. In general, the viscosityof grease decreases at high temperatures and increases at lowtemperatures. Generally, a grease material with a high viscosity at hightemperatures also has a high viscosity at low temperatures. As for thegrease used in an automobile which is exposed to various temperatureenvironments ranging from low to high temperatures, if a grease materialhaving a high viscosity is used to reduce noise, an operation load maybe induced at low temperatures. And, if a grease material having a lowviscosity is used to ensure operability at low temperatures, operationnoise may increase. As the result, the basic performance and durabilityof the related components may be compromised at high temperatures.

Accordingly, development of a grease material with a viscosity having adecreased dependency on temperatures is necessary for achieving animprovement of the performance and durability.

To minimize the change in viscosity as the temperature varies, costlyester base oils, silicone base oils or fluoroether base oils are oftenused as the base oil of grease. However, due to limitations placed byelectric parts and a high cost associated with the above base oils, apoly-α-olefin (PAO) synthetic base oil, which exhibits good costeffectiveness, is commonly used. Further, a method of increasingviscosity at high temperatures using polymer materials (such as,polyisobutylene, polybutylene (PB), etc.) has also been developed. U.S.Pat. No. 5,116,522 and U.S. Pat. No. 5,108,635 disclose a greasecomposition including PMA or a lithium-based thickener in a base oil.However, the grease composition has limited effects in maintainingdurability of the components used in the latest high-power,high-efficiency automobiles, and in reducing noise.

Korean Patent No. 513,625 discloses the use of PAO as a base oil, PMA asa polymer additive, and a lithium-based soap as a thickener for a greasematerial. Nevertheless, the resulting grease material is highlytemperature-dependent compared with that of the invention and offerslimited effects in improving durability of automotive parts undervarious temperature environments, maintaining operability of theautomotive parts, and reducing noise. The details are described inexamples infra. Korean Patent No. 135,414 discloses a grease compositionfor a ball joint. The grease composition includes a synthetic base oilcomprising a mixture of poly-α-olefin with a high viscosity index andpoly-α-olefin with a low viscosity index, a lithium-based soap as athickener, a PMA-based copolymer, and other supplementary additives. Thedurability at high temperatures and noise reduction effects areunsatisfactory despite that a relatively high weight percentage (25-50%)of the PMA-based copolymer is used. Satisfactory result has not beenobtained even after an antioxidant, an extreme pressure additive, etc.are added to improve the durability at high temperatures.

SUMMARY

The invention is based on the finding that use of a synthetichydrocarbon oil, instead of an ester oil, as a base oil and alithium-based thickener results in an improved service life of grease athigh temperatures while degradation is minimized at high temperatures.In particular, the invention is based on the finding that use of aPMA-based copolymer results in an improved operability at lowtemperatures and an improved durability at high temperatures, therebyproviding effects of reducing noise independent of a change intemperature.

In one embodiment of the invention, a PMA polymer material, instead of aPIB/PB polymer material, is added to a PAO-based synthetic base oil.When PAO with a viscosity of 4-6 cSt at 100° C. is used and when anasterisk type of PMA, instead of a linear type PMA, is used as thePMA-based copolymer, the dependency of the viscosity on temperature canbe minimized.

Accordingly, the invention provides a less temperature-dependentcold-proof grease composition for reducing noise. In certainembodiments, the grease composition of the invention includes asynthetic hydrocarbon oil as a base oil, a PMA-based copolymer, and alithium-based thickener.

In an aspect, the present invention provides a cold-proof greasecomposition for reducing noise comprising:

50-75 wt % of a base oil including a synthetic hydrocarbon oil;

10-30 wt % of a poly(methyl acrylate) (PMA)-based copolymer; and

10-20 wt % of a lithium-based thickener.

DETAILED DESCRIPTION

Reference will now be made in detail hereinafter to various embodimentsand examples of the invention. Further, the invention is illustrated inthe accompanying drawings and described below. While the invention isdescribed in conjunction with exemplary embodiments, it will beunderstood that the present description is not intended to limit theinvention to those exemplary embodiments. On the contrary, the inventionis intended to cover not only the exemplary embodiments, but alsovarious alternatives, modifications, equivalents and other embodiments,which may be included within the spirit and scope of the invention asdefined by the appended claims.

The invention provides a grease composition comprising a synthetichydrocarbon oil, instead of an ester oil, as a base oil, a PMA-basedcopolymer, and a lithium-based thickener. The grease composition of theinvention improves compatibility and adhesivity among various materialsincluding plastics, improves the service life of components to which thecomposition is applied under high temperatures and low temperatures, andensures a low-noise operation and an improved durability.

The base oil used in the invention can be any base oil commonly used ingrease. In certain embodiments, base oils having no effects on metals,plastics or rubbers may be used. Such base oils including, for example,a synthetic hydrocarbon oil, a mineral oil and a mixture thereof. In oneembodiment, a base oil consisting essentially of a synthetic hydrocarbonoil is used. The synthetic hydrocarbon oil may comprise poly-α-olefin(PAO) or a copolymer of α-olefin and olefin.

In one embodiment, the synthetic base oil of the invention has a dynamicviscosity of 4-6 cSt at 100° C. and a pour point of −50° C. or lower atlow temperatures. If the dynamic viscosity is lower than 4 cSt, thecomposition may evaporate easily, rendering the heat resistanceinsufficient. On the other hand, if the dynamic viscosity exceeds 6 cSt,torque and heat generation may increase.

According to the invention, the base oil may be used in an amount of50-75 wt %, or 58-72 wt %. If the content of the base oil is less than50 wt %, the grease composition may be solidified easily. And, if itexceeds 75 wt %, the composition may be liquefied at high temperatures.

In certain embodiments of the invention, a PMA-based copolymer is usedto improve the viscosity index and adhesivity of the base oil. Incertain embodiments, the PMA-based copolymer is added in an amount of10-30 wt %, or 13-24 wt %, of the entire grease composition. If thecontent of the PMA-based copolymer is less than 10 wt %, there would bean insufficient improvement of the viscosity index and adhesivityresulted from the addition of the PMA-based copolymer. And, if thecontent of the PMA-based copolymer exceeds 30 wt %, operability andnoise problems may occur at low temperatures due to an increasedviscosity.

In one embodiment, the PMA-based copolymer used in the invention is acopolymer containing a vinyl (—C═C—) group or a carbonyl (—COO—) group.The copolymer is represented by the following chemical formulas 1-4:

R₁COOR₂  (1)

R₃COOR₄COOR₅  (2)

R₆CH═CHR₇  (3)

R₈CH═CHR₉COOR₁₀  (4)

In the chemical formulas 1-4, R₁-R₁₀ is a hydrocarbon group, including,such as, a saturated aliphatic group, an unsaturated aliphatic group, anaromatic group or a substituent group. In a separate embodiment, thePMA-based copolymer of the invention is an asterisk type, not a lineartype. The asterisk type copolymer has a radial or star-shaped structureand, because of its structural characteristics, exhibits an increasedviscosity, an increased viscosity index and superior low-temperatureproperties when dissolved in oil. The improvement of the viscosity indexand adhesivity of the base oil resulted from the addition of thePMA-based copolymer means that the viscosity index of the base oilcomprising 20 wt % of the PMA-based copolymer is 200 or greater at 40°C. and 100° C. The viscosity index is calculated by measuring theviscosity of the base oil using a viscometer. The dynamic viscosity ismeasured according to KS M 2014.

In other embodiments of the invention, a lithium-based thickener is usedas the thickener of grease. The lithium-based thickener is used in anamount of 10-20 wt %, or 12-17 wt %, of the entire grease composition.If the content of the lithium-based thickener is less than 10 wt %,viscosity decreases at both high and low temperatures. And, if thecontent of the lithium-based thickener exceeds 20 wt %, viscositybecomes excessively high at both high and low temperatures, resulting insolidification of the composition.

The lithium-based thickener that may be used in the invention arerepresented by the following chemical formula 5:

(R₁₁—COOLi)_(n)  (5)

In the chemical formula 5, R₁₁ is a C₆-C₂₀ aliphatic hydrocarbon groupand n is an integer. If the number of carbons in R₁₁ is less than 6, thegrease composition may not be thickened sufficiently. And, if the numberof carbons in R₁₁ exceeds 20, the heat resistance of the greasecomposition may be poor.

The lithium-based thickener can be obtained by reacting lithiumhydroxide with a fatty acid compound. To ensure that no reactive fattyacid groups remain in the final product, lithium hydroxide and the fattyacid compound containing fatty acid groups in a mole almost equivalentto the lithium hydroxide are mixed. The lithium hydroxide and the fattyacid compound may be reacted in the base oil. Alternatively, apreviously synthesized fatty acid lithium-based soap thickener may bemixed with the base oil. In a specific embodiment, the former method isused to achieve a better grease stability.

The grease composition according to the invention may further compriseadditives commonly added to grease. Such additives including, such as,an extreme pressure additive, an antioxidant, a corrosion inhibitor,etc.

Further, if so desired, supplementary additives, such as, a rustinhibitor, a metal deactivator, and a viscosity index improver, may alsobe added.

An extreme pressure additive that is commonly used may be added toimprove the load resistance or extreme pressure properties. For example,the following compounds may be used as extreme pressure additives. Inone embodiment, the extreme pressure additives are organometalsincluding an organomolybdenum compound, such as, molybdenumdithiocarbamate, molybdenum dithiophosphate, etc.; an organozinccompound such as, zinc dithiocarbamate, zinc dithiophosphate, zincphenate, etc.; an organoantimony compound, such as, antimonydithiocarbamate, antimony dithiophosphate, etc.; an organoseleniumcompound, such as, selenium dithiocarbamate, etc.; an organobismuthcompound, such as, bismuth naphthenate, bismuth dithiocarbamate, etc.;an organoiron compound, such as, iron dithiocarbamate, iron octylate,etc.; an organocopper compound, such as, copper dithiocarbamate, coppernaphthenate, etc.; an organotin compound, such as, tin maleate,dibutyltin sulfide, etc.; an organic sulfonate, phenate or phosphate ofan alkali metal or an alkaline earth metal; an organometal compound ofgold, silver, titanium, cadmium, etc. In another embodiment, the extremepressure additives are sulfur compounds including a sulfide orpolysulfide compound, such as, dibenzyl disulfide, etc.; a sulfurizedoil; an ashless carbamate compound; a thiourea compound; and athiocarbonate compound, or the like.

Further, the extreme pressure additive can also be phosphate-based,which includes a phosphate compound, such as, trioctyl phosphate,tricresyl phosphate, etc.; or a phosphate ester compound, such as,phosphoric acid ester, phosphorus ester, phosphorus acid ester, etc.Also, a halogen-based extreme pressure additive can be used in theinvention. The halogen-based extreme pressure additives include, suchas, chlorinated paraffin, etc. In addition, a solid lubricant, such as,molybdenum disulfide, tungsten disulfide, graphite, PTFE, antimonysulfide, boron nitride, boron, etc., may be used. In a specificembodiment, the invention uses a dithiocarbamate compound or adithiophosphate compound as the extreme pressure additive.

The antioxidant of the invention may be selected from the group of anaging resister, an antiozonant, and an antioxidant that can be added torubber, plastics, lubricating oil, etc. For example, the antioxidant ofthe invention can be an amine compound, such as, phenyl-1-naphthylamine,phenyl-2-naphthylamine, diphenyl-p-phenylenediamine, dipyridylamine,phenothiazine, N-methylphenothiazine, N-ethylphenothiazine,3,7-dioctylphenothiazine, p,p′-dioctyldiphenylamine,N,N′-diisopropyl-p-phenylenediamine,N,N′-di-sec-butyl-p-phenylenediamine, etc., or a phenol compound, suchas, 2,6-di-tert-dibutylphenol, etc., or the like.

The corrosion inhibitor of the invention can be, for example, anammonium salt of a sulfonic acid, an organic sulfonate, carbonate,phenate or phosphate of an alkali or alkaline earth metal (such as,barium, zinc, calcium, magnesium, etc.), an alkyl or alkenyl succinicacid derivative (such as, alkyl or alkenyl succinic acid ester), apartial ester of a polyol (such as, sorbitan monooleate), a hydroxyfatty acid (such as, oleoylsarcosine), a mercapto fatty acid (such as,1-mercaptostearic acid) or a metal salt thereof, a higher fatty acid(such as, stearic acid), an ester of a higher alcohol (such as,isostearyl alcohol) with a higher fatty acid, a thiazole compound (suchas, 2,5-dimercapto-1,3,4-thiadiazole, 2-mercaptothiadiazole, etc.), animidazole compound (such as, 2-(decyldithio)-benzimidazole,benzimidazole, etc.), a disulfide compound (such as,2,5-bis(dodecyldithio)benzimidazole), a phosphate ester compound (suchas, trisnonylphenyl phosphite), a thiocarbonate ester compound (such as,dilauryl thiopropionate), or the like. Also, a nitrite compound may beused.

EXAMPLES

The invention is described in more detail through examples. Thefollowing examples are for illustrative purposes only. It will beapparent to those skilled in the art not that the scope of thisinvention is not limited by the examples.

Examples 1-5 and Comparative Examples 1-7

A cold-proof grease composition for reducing noise was prepared bymixing a base oil, a lithium-based thickener and a PMA-based copolymeras described in Table 1. Poly-α-olefin oil (ExxonMobil, viscosity: 4-6cSt at 100° C.), which is a synthetic hydrocarbon oil, was used as thebase oil. The lithium-based thickener was obtained by reacting lithiumhydroxide with 12-hydroxystearic acid. And, asterisk type PMA (Lubrizol,viscosity: 500-800 cSt at 100° C.) was used as the PMA-based copolymer.

TABLE 1 Examples Comparative Examples (wt %) 1 2 3 4 5 1 2 3 4 5 6 7Synthetic hydrocarbon oil 63.3 71   59   62.2 64.9 73   55.5 66.3 60.354.3 56.3 62.1 Methacrylate polymer A 22.2 14.5 26.5 21.8 22.8  9.2 32  22.2 22.2 — — — Methacrylate polymer B — — — — — — — — — 24.6 —Methacrylate polymer C — — — — — — — — — 25.5 — Methacrylate polymer D —— — — — — — — — — — 23.4 Li-soap 14.5 14.5 14.5 16   12.5 14.5 14.5 9 21.5 14.5 14.5 14.5

Test Example

Penetration of the base oil was measured according to ASTM D217.

Apparent viscosity of grease was tested according to DIN 51810.

Low-temperature torque of grease was tested according to KS M 2130.

Operation noise was tested by applying the grease composition of theinvention or samples prepared according to the existing art to a doorlatch. Noise is compared.

TABLE 2 Examples Comparative Examples (wt %) 1 2 3 4 5 1 2 3 4 5 6 7Penetration 290 280 285 267 310 295 290 325 245 294 289 295 Apparentviscosity (25° C.) 2.6 2.3 2.8 2.8 2.4 1.9 3.2 1.7 2.9 2.3 2.5 2.1Apparent viscosity (−40° C.) 14.9 13.2 17.2 18.6 13.0 12.4 22.3 11.722.0 35.0 40.0 37.0 Low- startup 2379 2156 2689 2415 2015 1956 3318 18283508 5527 6082 5723 temperature revolution 856 795 902 895 752 698 1084611 1052 1858 2130 2017 torque (−40° C.)

Table 2 shows the result of penetration measurement, change in viscosityat room temperature and low temperature, and low-temperature torque ofthe grease compositions of Examples 1-5 and Comparative Examples 1-7. Itcan be seen that the compositions of Examples 1-5 do not showsignificant viscosity increase at low temperatures, compared to theprior art samples at low temperatures.

TABLE 3 Existing The present art invention Improvement Viscosity (Pa ·s) room 2.02 2.6 0.58 (29%)   temperature low 5.96 14.9 8.94 (150%) temperature Operation Latch closed 72.1 67.1 5.0 (6.9%) noise lockingopen 78.0 75.1 2.9 (3.7%) Ac- closed 71.0 65.2 5.8 (8.1%) tuator open64.5 60.6 3.9 (6.0%)

The grease composition according to the existing art containspoly-α-olefin oil as the synthetic hydrocarbon oil and a lithium-basedthickener obtained by reacting lithium hydroxide with 12-hydroxystearicacid. A low-viscosity poly-α-olefin oil was used to ensure goodlow-temperature properties. Although the grease composition according tothe existing art showed good low-temperature properties, operation noiseoccurred because of the low viscosity. In contrast, the greasecomposition according to the invention showed an improvement in reducingoperation noise, as its viscosity was higher than that of the prior artcomposition both at the room temperature and low temperatures. Table 3compares the results of the measured viscosity and operation noise ofthe prior art grease compositions and the grease compositions of theinvention at the room temperature and low temperatures. It was expectedthat the grease composition of the invention is effective in reducingnoise as it has an increased viscosity compared to that of the prior artcomposition. This noise-reduction effect of the grease composition ofthe invention was confirmed in latch locking and actuator operation,when the operation noise was tested for a door latch.

The grease composition according to the invention comprises a synthetichydrocarbon oil, instead of ester oil, as a base oil. The synthetichydrocarbon oil is effective in preventing brittleness (cracking) andenhancing durability without incurring adverse effects on plastics andrubbers. Further, the addition of a lithium-based thickener improvesmechanical stability and durability. The addition of a PMA-basedcopolymer, which improves viscosity index and adhesivity of the baseoil, helps to prevent degradation of grease, and greatly improves theservice life of grease at high temperatures and the lubricatingperformance at very low temperatures. Furthermore, the greasecomposition ensures low noise and vibration under all temperature andhumidity conditions to which high-power, high-efficiency automobiles maybe exposed.

The invention has been described in detail with reference to specificembodiments thereof. However, it will be appreciated by those skilled inthe art that various changes and modifications may be made in theseembodiments without departing from the principles and spirit of theinvention, the scope of which is defined in the appended claims andtheir equivalents.

What is claimed is:
 1. A cold-proof grease composition for reducingnoise comprising: 50-75 wt % of a base oil comprising a synthetichydrocarbon oil; 10-30 wt % of a poly(methyl acrylate) (PMA)-basedcopolymer; and 10-20 wt % of a lithium-based thickener.
 2. Thecold-proof grease composition for reducing noise according to claim 1,wherein the synthetic hydrocarbon oil is an aliphatic hydrocarbon oilwith a viscosity of 4-6 cSt at 100° C.
 3. The cold-proof greasecomposition for reducing noise according to claim 2, wherein thealiphatic hydrocarbon oil comprises poly-α-olefin (PAO) or a copolymerof α-olefin and olefin.
 4. The cold-proof grease composition forreducing noise according to claim 1, wherein the PMA-based copolymer isan asterisk type having a radial or star-shaped structure and has adynamic viscosity of 500-800 cSt at 100° C.
 5. The cold-proof greasecomposition for reducing noise according to claim 4, wherein thePMA-based copolymer comprises a styrene-alkyl methacrylate copolymer ora styrene-alkyl methacrylate copolymer.
 6. The cold-proof greasecomposition for reducing noise according to claim 1, wherein thelithium-based thickener is obtained by reacting lithium hydroxide with12-hydroxystearic acid or by reacting lithium hydroxide with ahydrogenated castor oil.
 7. The cold-proof grease composition forreducing noise according to claim 1, further comprising 3 parts byweight of the grease composition of one or more additive selected from agroup consisting of an extreme pressure additive, an antioxidant, acorrosion inhibitor, a rust inhibitor, a metal deactivator and aviscosity index improver.